A rechargeable battery, also known as a storage battery, is a group of two or more secondary cells. These batteries can be restored to full charge by the application of electrical energy. In other words, they are electrochemical cells in which the electrochemical reaction that releases energy is readily reversible. Rechargeable electrochemical cells are therefore a type of accumulator. They come in many different designs using different chemicals. Commonly used secondary cell chemistries are lead and sulfuric acid, rechargeable alkaline battery (alkaline), nickel cadmium (NiCd), nickel metal hydride (NiMH), lithium ion (Li-ion), and lithium ion polymer (Li-ion polymer).
Rechargeable batteries can offer economic and environmental benefits compared to disposable batteries. Some rechargeable battery types are available in the same sizes as disposable types. While the rechargeable cells typically have a higher first cost than disposable batteries, rechargeable batteries can be discharged and recharged many times. Proper selection of a rechargeable battery system can reduce toxic materials sent to landfill disposal compared to an equivalent series of disposable batteries. Some manufacturers of NiMH type rechargeable batteries claim a lifespan up to 3000 charge cycles for their batteries.
Lead-acid batteries, invented in 1859, are the oldest type of rechargeable battery. The lead-acid batteries have a large energy-to-weight ratio and a correspondingly large energy-to-volume ratio, and therefore are able to supply high surge currents means while maintaining a relatively large power-to-weight ratio. These features, along with their low cost, make them attractive for use in automobiles, to provide the high current required by automobile starter motors.
The Lithium-ion (“Li-ion”) battery is another type of rechargeable battery in which a lithium ion moves between the anode and cathode. The lithium ion moves from the anode to the cathode during discharge and from the cathode to the anode when charging. Lithium ion batteries are commonly used in consumer electronics. They are particularly advantageous for portable electronics because the Li-ion batteries have one of the best energy-to-weight ratios, have no memory effect, and have a slow loss of charge when not in use. In addition to uses for consumer electronics, lithium-ion batteries are growing in popularity for defense, automotive, and aerospace applications due to their high energy density.
The Li-ion batteries have a significantly better power-to-weight ratio than the lead-acid batteries and would therefore be a better choice where traditional lead-acid applications (such as automotive batteries) are used. However, Li-ion batteries are not widely used in such other systems because of two problems. The first problem is that the Li-ion batteries need individual charge control, and the second problem is that the Li-ion batteries require cell balancing. Li-ion battery cells require careful charge management to ensure that significant over-charge and over-discharge does not occur. This is because Li-ion batteries possess an extreme sensitivity to overcharging and over-discharging not found in most other types of batteries.
Cell balancing is defined as the application of differential currents to individual cells (or combinations of cells in a battery pack) in a series string. Normally, cells in a series string receive identical currents. A battery pack therefore requires additional components and circuitry to achieve cell balancing. Battery pack cells are balanced when all the cells in the battery pack meet two conditions:
1. If all cells have the same capacity, then they are balanced when they have the same relative State of Charge (SOC.) In this case, the Open Circuit Voltage (OCV) is a good measure of the SOC. If, in an out of balance pack, all cells can be differentially charged to full capacity (balanced), then they will subsequently cycle normally without any additional adjustments.
2. If the cells have different capacities, they are also considered balanced when the SOC is the same. But, since SOC is a relative measure, the absolute amount of capacity for each cell is different. To keep the cells with different capacities at the same SOC, cell balancing must provide differential amounts of current to cells in the series string during both charge and discharge on every cycle.
Because of the need for individual charge control and cell balancing, the Li-ion batteries are not preferred in high volume systems (e.g., automotive batteries) despite offering a higher power-to-weight ratio than the batteries used in prior art (e.g., the lead acid batteries). Additionally, Li-ion batteries are highly susceptible to permanent damage in the events of high temperature, over-charging, and over-draining. Therefore, these parameters need to be carefully monitored.